Method of heat treating metals by electrolytic processes



tory results.

United States Patent )fiFice 2,953,672 Patented Sept. 20, 1960 Adolf Wisken, Hamburg, Germany. J. J. Blair, 15 Madison Ave., New York, N.Y.), and Jack J. Bauer, 518 Manhasset Woods Road, Manhasset, NY.

Filed Oct. 29, 1956, Ser. No. 618,792 .7 Claims. (Cl. 219-71) This invention relates to a method of heating metals, by electrolytic processes. More specifically it relates to a process whereby a metal is treated with a controlled heat to further particular objectives, to be hereinafter set forth. a

For many years it has been found necessary to heat metals in order to work with them or process them. They must be heated forforming, drawing, case hardening, tempering and many other operations inherent in the fields of their use. v In the past many heating processes have been employe none of which, however, have delivered satisfac- In one-of these processes flame is applied directly to the metal, but the resultant heat is not uniform, causing, in many cases, uneven stresses in the metal. Further, where flame is applied to a metal of uneven contour, the peaks on the metal tend to become overheated While the depressions do not receive suflicient heat.

Likewise, high frequency heating does not give the desired results, since the depth of penetration of the heat .varies with the contour of the metal surface. It is also worthy of note that heating processes utilizing high frequency are extremely expensive.

Gas or oil fired furnaces are expensive to construct and operate and are not satisfactory in achieving the desired results. Their use is time consuming and tends to covercthe metals with harder and higher oxide coatings which are difficult to remove.

Our invention remedies the difiiculties encounteredin the old methods and provides a process which is econo cal, fast and, most important, highly eifective.

Basically the process involves electrolysis in which controls are effected such that the desired results obtains. The metal to be heated is utilized as the cathode in an electrolytic apparatus. A current of a required amount, at a steady voltage, is applied between an anode and cathode to start the process through the electrolytic bath in order to form a heavy layer of minute bubbles enclosed in complete vapor envelopes to form on said cathode which offer a plurality of extremely high resistance paths to the current and thereby cause the metal to heat very rapidly.

It is an object of the invention to provide a process for heating metals which is fast and eificient.

It is. another object of the invention to provide a process for heating metals such that an oxide coating will not form on the surface of the metals.

It is also an object of the invention to provide a process whereby both smooth and uneven surfaces of a metal may be evenly heated.

It is a further object of the invention to provide a process for heating metals which will be equally effective in small or large scale use.

It is a still further object of the invention to provide a process for heating metals which may be readily incorporated in a production line type of operation.

With these and other objects in mind reference is had to the attached sheet of drawings wherein:

Fig. 1 illustrates the basic reactions of the process;

Fig. 2 shows typical curves illustrating currents resulting from variations in electrolyte concentration;

Fig. 3 shows typical curves illustrating current density resulting from variations in electrolyte concentration;

Fig. 4 shows typical curves illustrating voltage to current relationships in normal electrolytes of HCL and Na CO and Fig. 5 shows typical curves illustrating current density resulting from changes in surface area of a metal exposed to an electrolyte of constant concentration.

If an electric current is passed through a particular electrolyte, under definite conditions, the normal electrolysis process can be altered so that an intensive heating occursat one of the electrodes. This heating will occur at the instant when the current density at the cathode is greater than the current density at the anode; this difierence in density results from the lowered conductivity of the gaseous layer formed at the cathode.

The presence of the gaseous layer surrounding the cathode may be understood by the following analogy; if a drop of water is placed on a red hot metal plate, the drop assumes a flattened spherical shape and jumps about on the plate without being evaporated immediately. If the heated surface is cooled to a definite temperature, however, there occurs a sudden evaporation of the water drop. This phenomenon is explained by the fact that the drop of water is surrounded by a gaseous layer which serves as a heat insulator and thus hinders evaporation. When the temperature is lowered the gaseous layer condenses and the water evaporates.

In the process which is the subject of the instant application an electric current is passed through an electrolytic bath. As a result of this passage of current, bubbles are formed around the metal to be heated which is immersed in the bath and which serves as a cathode.

The heavy layer of minute bubbles which form on the cathode in normal operation and which are completely enclosed in a vapor envelope oifer a plurality of extremely high resistance paths to the current flowing in the electrolyte and thus cause the metal to heat very rapidly.

' The transient period from the application of the current to the formation of vapor is extremely short, being about 0.0016 seconds. The heating of the cathode follows immediately. It is worthy of note that this latter step will not occur if the minute bubble production of the electrolyte is insuflicient due to low voltage and current values. Therefore, the formation of the vapor envelopes will be retarded or stopped and the process will not start; and the minute bubbles that have formed may be condensed and dissipated by the electrolyte which remains comparatively cool at all times.

The electrolyte is not heated to any substantial degree, due to the gaseous layer acting as an insulator between the cathode and the electrolyte. Likewise the cathode does not lose heat since it is insulated by the gaseous layer as long as the current remains on.

Fig. 1 illustrates our process in its most basic form. A metal tank 10 contains an electrolytic bath 11. Immersed in the bath 11 is a metal 12 which is to be heated. The. metal 12 is the cathode in our process and the tank 10 is the anode. On the passage of current (source not shown) through the bath 11, from the anode to the cathode, minute bubbles 13 are formed on the cathode. The heat resulting from the continued flow of current forms a vapor envelope 14 around the bubbles, which vapor envelope acts as a resistance to a further flow of current. The vapor envelopes otter a plurality of extremely high resistance paths to the current and thereby cause the metal to heat very rapidly.

Direct current has been found to be the most effective in producing the required heating action. However, the process is not limited to this form of current flow. A sustained heating action may be obtained by the use of rectified alternating current with a time lag between the cycles. It should also be noted that rectified alternating current, direct current with a superimposed high frequency alternating current and direct currents of varying amplitudes may be employed in the process depending on the character of the metal to be heated, the type of heating operation and the character of the electrolyte.

The process may be controlled by various factors. A change in either the concentration of the electrolyte, the voltage or the current will produce changes in current density, which changes will produce variations in the amount of heat at the cathode and in the time required to achieve such heat.

In Fig. 2 an iron rod having a 10 mm. diameter is assumed as being immersed in a sodium carbonate electrolyte at a constant depth. The voltage is constant at 220 volts. By varying the concentration of the electrolyte, there will be a related change in the current, as measured in amperes. Where the rod is immersed to a depth of mm., the current will increase from 9 amperes to 20 amperes as the concentration of the electrolyte is increased from 2% to 28%.

Fig. 3 illustrates graphically the variations in current density due to changes in the concentration of the electrolyte. As in Fig. 2, an iron rod, having a mm. diameter, is immersed at a constant depth in a sodium car'- bonate electrolyte. The voltage is constant at 220 volts. A sharp increase in current density follows an increase in the concentration of the electrolyte up to a point at which it substantially levels oil. When the rod is immersed to a depth of 5 mm. in the electrolyte, the current density increases from 2.25 amperes/cm. to 8 amperes/ cm. as the concentration of the electrolyte increases from 2% to 18%. At this point the current density substantially levels off and remains at about 8 amperes/cm. with further increases in electrolyte concentration.

The graphs only illustrate the change in current and current density resulting from changes in concentration of a sodium carbonate electrolyte. It may be pointed out, however, that the curves will he basically the same for all hydrogen producing electrolytes.

The heat the cathode is subjected to and the time required to reach this heat may be completely controlled by varying the concentration of the electrolyte. An iron rod, as in Fig. 2, having a 10 mm. diameter and being immersed to a depth of 5 mm. in a sodium carbonate electrolyte, at a constant 220 volts may be treated with varying temperatures by merely changing the concentration of the electrolyte. The time required to reach these temperatures will also vary with the concentration of electrolyte. With a 2% concentration a current of 9 amperes will flow producing in 3 to 4 seconds a temperature of 438 C. at the cathode. A temperature of 500 C. may be produced in l to 2 seconds by utilizing a 28% concentration of electrolyte causing a ampere current flow.

Variations in voltage will effect the current and consequently the heating effect of the process. With low voltages, the normal process of electrolysis occurs and there is an evolution of gas. If the voltage is raised there is a characteristic crackling at the cathode and the surrounding gaseous layer begins'to discharge small gaseous bubbles which contain molten metal and burning gases. This discharge is only present, however, where the rod is not completely immersed in the electrolyte. At this point, there occurs over the entire immersed area of the cathode, a kind of phosphorescent discharge of light and the current drops sharply. By a further increase of voltage the heating process begins.

Fig. 4 illustrates graphically the effect of voltage changes on the heating process where normal electrolytes of hydrochloric acid (HCL) or sodium carbonate (Na CO are used. From a point A up to point B the current increases on raising the voltage. This is the normal electrolysis. A continued increase in voltage will cause a sharp current drop to point C, at which place uneven light phenomena are produced. Further increases in voltage from point C produce a heating of the cathode, which continues to point D. Once the heating effect is started the voltage can be lowered and the process will continue, although with lessened intensity. This, thereby, providing the necessary means of controlling temperature and duration of treatment.

To illustrate the effect of voltage changes on the heating process, assume an iron rod having a 10 mm. diameter is immersed to a depth of 5 mm. in a 28% sodium carbonate electrolyte. At 220 volts 20 amperes of current will flow through the electrolyte producing a temperature of 500 C., in l to 2 seconds, at the cathode. An increase in voltage to 250 volts will increase the current flow to 25 amperes and in 1 to 2 seconds will produce at the cathode a temperature of 550 C.

Increases and decreases in current, while the other variables in the process remain constant, will also affect the current density and consequently the heating process.

A change in any one or a combination of the variables, voltage, current or concentration of electrolyte, will produce ditferences in the heating effect, as to time and intensity.

It should be pointed out that changes in the surface area of the cathode exposed to the electrolyte will affect the current density and resultant heating efiect. In Fig. 5 an iron rod is immersed in a sodium carbonate electrolyte of 10% concentration. The voltage is constant at 220 volts. It can be seen, that as a rod having a specific diameter is immersed to an increasing depth in an electrolyte, there is an increase in current density. When the depth of immersion reaches a certain point, depending on the diameter of the rod, the current density begins to decrease and continues to do so, with lessened density, as the depth ,of immersion becomes greater. It can also be seen, that as the diameter of the rod increases the increase and subsequent decrease in current density become smaller.

Since the surface area of the metal to be treated will vary with the operation, changes in electrolyte concentration, voltage or current may be necessary to achieve the desired heat.

Generally, most electrolytes known to the art may be employed with this process. As aforementioned, the electrolyte must be capable of producing sufficient hydrogen so that the vapor formed around the bubbles will not condense back into the electrolyte. It should be added, that the choice of electrolyte to be used for heating may be limited by the character of the metal and the treatment to be applied.

We have found the following electrolytes preferable in this process, due to their all purpose nature:

Electrolyte N0. 1

6 to 30% NaOH Bal nee wt '10 to 20% KNo a Electrolyte N0. 2

6 to 30% NaOH 10% to 25% }Balance water Electrolyte N0. 3

6 to 30% NaOH 10 to 20% Na 'CO }-Balance water 3 shown in Figs. 2 and 3 the higher the concentration of the electrolyte the more rapid will be the heating efiect, with accompanying higher temperatures, available where necessary.

The process, as described above, is capable of replacing the old methods of heating metals. It is effective, economical and of such character that it may be employed in many diversified types of operations.

In respect to economy, the equipment required may be of a permanent character, not requiring frequent repairs or the replacement of parts. The consumption of electric power is relatively low compared to the old methods of heating metals, thus reducing costs and presenting a safety factor.

The process is extremely effective, providing all the benefits of the old methods, with additional factors required by advancing technology. In this process metals having smooth or uneven surfaces, may be heated with equal success, since the bubbles and subsequent vapors will adhere to, and follow the contour of the metal's surface. An even heat will be distributed throughout the surface otthe metals, reducing strains and penetrating the metal to an even depth at all places.

The oxide coating on the metals that results from most of the prior art heating methods is absent from our method since the metals are heated under the electrolyte in an atmosphere free of air.

Theprocess reduces the number of steps where the operation requires the chilling of the metal after it has been heated. On completion of the heating process, the passage of current may be discontinued and the electrolyte, which has remained cool, will chill the metal. The metal may also be chilled while the current continues to flow, by withdrawing the heated metal from the electrolytic bath through an oil layer, which has been floated on the surface of the electrolyte for producing oil hardening.

The most important feature of the process is its flexibility. It may be incorporated into a production line operation, as just another step in the operation, thus eliminating the necessity of separately heat treating metals. For example, wire or sheet metal may be case hardened, heated for drawing, or heat treated for other purposes, by merely passing the metal through the electrolytic bath in electrical association with the cathode. It would not be necessary to slow down the production line to individually heat separate strips of wire or sheets of metal.

The process of the invention may be utilized for producing ball bearings. A rod, as in Fig. 1, of a required diameter, may be reduced to molten drops by the process. As the drops fall toward the bottom of the bath container they are chill hardened by the cool electrolyte and formed by the liquid pressure of the electrolyte.

Thus, among others, the several objects of the invention as aforenoted are achieved. Obviously, numerous changes in method may be resorted to without departing from the spirit of the invention as defined by the claims.

We claim:

I. A process for heating metals comprising immersing the metal to be treated a preselected electrolytic bath and causing a current at a given voltage to flow in such manner that the metal functions as a cathode, thereafter causing a heavy layer of minute bubblesenclosed in complete vapor envelopes to form on said cathode, which envelopes otter a plurality of extremely high resistance paths to the current and thereby cause the metal to heat very rapidly, at said given voltage said current decreases when said vapor envelopes form about said bubbles, and said voltage is applied at a suitable vgiven value to start the process, then may be dropped to a suitable value to continue and closely control the heating of the metal.

2. In a process as in claim 1 wherein the current varies as the voltage, and when the voltage is raised to start the heating the current follows, and conversely when the voltage is dropped to control the process the current follows.

3. A process for heating metals according to claim 1 wherein the concentration of the compounds in the electrolytic bath further controls the heating of the metal.

4. A process for heating metals according to claim 1 wherein a tank contains the electrolytic bath and functions as the anode.

5. A process for heating metals according to claim 1 wherein the electrolytic bath is comprised of 8% to 30% NaOH, 10% to 20% KNO; and water.

6. A process for heating metals according to claim 1 wherein the electrolytic bath is comprised of 6% to 30% NaOH, 10% to 15% HCL and water.

7. A process for heating metals according to claim 1 wherein the electrolytic bath may be comprised of 6% to 30% NaOH, 10% to 20% Na CO and water.

References Cited in the file of this'patent 

1. A PROCESS FOR HEATING METALS COMPRISING IMMERSING THE METAL TO BE TREATED IN A PRESELECTED ELECTROLYTIC BATH AND CAUSING A CURRENT AT A GIVEN VOLTAGE TO FLOW IN SUCH MANNER THAT THE METAL FUNCTIONS AS A CATHODE, THEREAFTER CAUSING A HEAVY LAYER OF MINUTE BUBBLES ENCLOSED IN COMPLETE VAPOR ENVELOPES TO FROM ON SAID CATHODE, WHICH ENVELOPES OFFER A PLURALITY OF EXTEREMLY HIGH RESISTANCE PATHS TO THE CURRENT AND THEREBY CAUSE THE METAL TO HEAT VERY RAPIDLY, AT SAID GIVEN VOLTAGE SAID CURRENT DECREASES WHEN SAID VAPOR ENVELOPES FORM ABOUT SAID BUBBLES, AND SAID VOLTAGE IS APPLIED AT A SUITABLE GIVEN VALUE TO START THE PROCESS, THEN MAY BE DROPPED TO A SUITABLE VALUE TO CONTINUE AND CLOSELY CONTROL THE HEATING OF THE METAL. 